Without limiting the scope of the invention, its background is described in connection with the manufacturing of bipolar transistors, as an example.
Heretofore, in this field, in bipolar mesa transistors, the base layer extends outside the area of the active device region to enable contacting of the base. This area is called the extrinsic base region, outside the active device region. For collector-up transistors, that portion of the emitter-base junction in the extrinsic base area reduces the current gain, since minority carriers injected into the base in this area do not contribute to the collector current. Unless the emitter-base junction in the extrinsic base area is rendered inactive, the current gain may be unacceptably low.
Also, the extrinsic base region adds to the junction capacitance between the base layer and the layer beneath the base (the emitter, in collector-up devices). Since junction capacitance limits the high-frequency performance of bipolar transistors, great effort has been spent on reducing the junction capacitance.